Geology Reference
In-Depth Information
Microcodium
Glück 1912 (Pl. 128/4)
Morphology
: The name
Microcodium
refers to struc-
tures consisting of aggregates of up to 1 mm long pris-
matic to pyramidal monocrystalline laths of calcite, with
a central cavity in each lath. Depending on the arrange-
ment of the basic units, two types can be distinguished:
(a) the units may be radially distributed around a cen-
tral axis producing a structure resembling an ear of
maize, or (b) the units are stacked parallel to each other,
along axes that branch in successive planes, thus form-
ing thick crusts.
Distribution
: First described from Miocene depos-
its,
Microcodium
and closely related taxa (Micro-
codiaceae) have been found in carbonates as old as the
Devonian.
Many authors consider
Microcodium
to be restricted
to the Cenozoic, associated with lacustrine environ-
ments and indicative of pedogenic origin.
Interpretation
: Older records explained
Micro-
codium
as a product of per descensum contamination.
However, Paleozoic occurrences demonstrate that
microcodiaceans thrived in shallow-marine and supra-
tidal as well as in lacustrine environments (Nassichuk
et al. 1986).
Microcodium
has been assigned to green
alga, 'blue-green alga' or bacteria (Mamet and Roux
1982).
Some authors have suggested a diagenetic origin.
Non-marine occurrences in paleosols and caliche were
explained as products of calcite-secreting filamental
bacteria living in symbiosis with other organisms
(fungi) within the soil (Esteban 1974; Bodergat 1974)
or as calcified roots (Klappa 1978; Jaillard et al. 1991).
Microtubus
Flügel 1964 (Pl. 99/3)
Morphology:
Small straight or slightly bent hollow
tubes with thin walls of microcrystalline calcite. One
end of many tubes appears to be enclosed within mi-
critic matrix. The tubes occur isolated or more com-
monly in aggregates. Tube diameter 0.05 to 0.20 mm,
mostly 0.10 mm.
Distribution:
Abundant in Late Triassic (Norian and
Rhaetian) reef limestones, often associated with mi-
critic spongiostromate crusts.
Microtubus
is known
from all parts of the Tethys. It characterizes the central
reef facies.
Interpretation:
The genus was tentatively assigned
to serpulid worms but may have cyanobacterial affinity.
Nuia
Maslov 1956
(Pl. 98/1)
Morphology:
The genus is characterized by cylin-
drical bodies consisting of transparent fibrous calcite
crystals growing in zones around micritic centers.
Plate 98 Paleozoic Microproblematica
Shown on this plate are common Paleozoic thin-section fossils whose systematic assignment is disputed.
1
Nuia.
The arrows point to longitudinal and cross sections of cylindrical bodies characterized by transparent fibrous calcite
crystals growing in zones around micritic centers. The fossil is interpreted as a bacterial or cyanobacterial growth form.
Nuia
is a common constituent of Late Cambrian and Early Ordovician reef mounds. Ordovician: Antarctica.
2
Halysis
. The fan-shaped structure exhibiting palisades and chains in cross-sections is now regarded as a cyanophycean
taxon. Early Ordovician (Arenigian): Megaconglomerate unit of Cow Head Peninsula, western Newfoundland.
3
Koivaella
. Small hollow tubes with well-defined micritic walls are common microfossils in Late Paleozoic reef and
platform carbonates. They are interpreted as cyanobacteria or green alga. Early Permian (Sakmarian): Forni Avoltri,
Carnia, Italy.
4
Pseudovermiporella sodalica
Elliott characterized by attached tubes with perforated walls. The fossil is considered to be
a dasyclad alga, a foraminifera or of cyanobacterial origin.
Pseudovermiporella
is abundant in Late Permian shelf carbon-
ates. Permian: Taurus Range, Turkey.
5
Tubiphytes obscurus
Maslov, encrusted by
Archaeolithoporella
(A). The longitudinal section shows the segmentation of
the fossil, the spar-filled cavities (arrows) and the surrounding spider-like network. The current explanation of Late
Paleozoic
Tubiphytes
is a commensalism of different organisms responsible for the internal cavities and tubes and cyano-
bacteria or other bacteria responsible for the network (see Fig. 10.60).
Tubiphytes obscurus
is characterized by tubes in
each segment. Middle Permian (Murgabian): Sosio, Sicily, Italy.
6
Tubiphytes obscurus
Maslov. Cross section. Same locality as -> 5.
7
Palaeoaplysina
Krotow, a common Late Carboniferous to Early Permian reef-building organism. The calcareous plates
are characterized by an internal differentiation and a canal system. The fossil is regarded as an alga or a sponge. Early
Permian (Asselian): Duvansk reef complex, central Urals, Russia.
8
Archaeolithoporella hidensis
Endo, an abundant organism in Permian reefs acting as encruster, binder and frame builder.
The characteristic criteria are couplets of micritic and microsparitic or sparry irregularly undulated layers. Middle Per-
mian (Wordian): Oman.
-> 2: Pohler and James 1989
